With the continuing drive for improved engine performance, fuel consumption, and exhaust emissions, it is becoming increasingly important to precisely control the timing and quantity of fuel injected into the cylinder. In electronically controlled fuel injection systems, injection can be easily timed with respect to the piston top dead center position for all conditions of speed and load. The duration of injection is determined in terms of crankshaft degrees and, for any given fuel pressure, is varied to change the quantity of fuel injected into the combustion chamber for each combustion cycle.
Optimizing engine performance and emissions requires that injection occur over a certain number of crankshaft degrees, which will vary depending on engine speed, load, and other conditions. However, because of system inadequacies inherent in known diesel and stratified charge engines, the quantity of fuel required necessitates that the duration of injection be greater than the optimum number of crankshaft degrees. Thus, injection has traditionally been advanced or retarded and extended to run longer than the optimum number of crankshaft degrees. However, when injection is begun too early in the combustion process, several problems result. For a stratified charge engine, the combustion process begins to change its fundamental characteristics, behaving more like a homogenous-mixture engine and losing the benefits of stratification. For diesels, too much fuel will be present when combustion begins and will result in the "knocking" often associated with diesel engines. Additionally, the fuel droplets will tend to agglomerate to form larger fuel droplets and too much fuel will be deposited on (i.e., wet) the cylinder walls, resulting in poor combustion and increased emissions. On the other hand, if injection is extended to run too late in the combustion cycle, the fuel at the tail end of injection will not have the time needed to properly mix and burn, resulting in smoke-limited output, high fuel consumption, and high energy losses to the exhaust and engine coolant. These situations become worse at higher engine speeds because the time it takes to rotate through the optimum number of crankshaft degrees becomes less.
To properly accommodate those particular conditions of speed, load, and other factors that require large quantities of fuel without sacrificing the optimum timing and duration of injection, fuel injection systems have been developed which vary the pressure of the fuel to thereby vary the rate at which fuel enters the chamber. One such system is commonly referred to as the Cummins PT system and is described in Diesel Engine Catalogue, Vol. 20, 1955. The Cummins PT systems uses a low pressure common rail with camshaft-driven injectors generating the high pressure. The low pressure is controlled by a throttle to thereby adjust the amount of fuel filling the injectors and, therefore, the quantity of fuel injected into the cylinders.
A second type of system which provides control of the pressure of the fuel being injected into the chamber is disclosed in U.S. Pat. No. 4,757,795, issued Jul. 19, 1988 to W. W. Kelly. That system utilizes what is commonly referred to as a rotary type distributor pump. Fuel is supplied at low pressure to the distributor pump, which pressurizes the fuel using cam-driven plungers. The high pressure fuel is supplied via a fuel distributor rotor to an outlet that feeds the fuel to one of the fuel injectors. Like the Cummins PT system, this system utilizes a low pressure fuel supply with the high pressure being generated individually for each injector.
A third type of system uses in-line or jerk-type pumps. Fuel injection systems using these types pumps have one pump per fuel injector. These pumps are camshaft-driven reciprocating-displacement pumps supplied with fuel from a low pressure fuel supply. Each pump produces a high pressure charge of fuel that is supplied to its associated hydraulic injector.
Yet a fourth such system is commonly known as the Cooper-Bessemer system and has been used in marine and large industrial applications. That system utilizes piston pumping elements to generate high pressure in a common rail. A pressure regulating valve that is controlled in accordance with speed and load is used to vary the pressure from about 3,200 to 13,600 psi. Fuel is gated from the common rail to the injectors by fuel doors. The fuel doors are cam-driven check valves that permit control of the timing and quantity of fuel provided to its associated injector. The Cooper-Bessemer system is described in Diesel Engine Catalogue, Vol. 13, 1948.
None of the aforementioned fuel injection systems provide complete and independent control of the pressure, timing, and duration of injection which is necessary for achieving optimum engine performance and emissions control. Although the Cooper-Bessemer system permits control of both the timing and duration of injection, it does not permit them to be independently controlled. That is, advancement of the beginning of injection is necessarily accompanied by lengthening of the duration of injection. Moreover, the Cooper-Bessemer system involves a length of fuel line running between the fuel doors and the injectors. These lengths of fuel line reduce the amount of spill control and introduce sonic disturbances resulting from the fluid dynamics of the fuel flowing in the lines.
Other than simply controlling the rate of injection (i.e., pressure) from one injection event to another, it is also desirable to be able to vary the injection rate over the course of a single injection. In the jerk-type pumps noted above, this is done by designing the profile of the cam in accordance with the desired injection rate profile. A rough form of controlling the injection rate has also been done by pilot injection. For example, pilot injection has been accomplished using a large piezoelectric stack to generate the pressure needed to pump the fuel through the hydraulic injectors and into the cylinder. The piezoelectric stack was given an initial pulse to inject a small quantity of fuel and, after a small delay time, once autoignition of the fuel was imminent, was again operated to ram fuel into the cylinder for combustion. However, this pilot injection system required an impracticably large piezoelectric stack and only provided an initial pulse of fuel rather than a controlled rate of injection.